The problem with most of the sequels is that they don't really establish anything new. JOTB for example basically is just a trip to the mountain and back and the lesson is something thar was already covered in the first movie. Also, they have usually lacked Bluth's overwhelming emotional drive. The ideas aren't bad per se as you have showed in Shorty's Dark Past. LBT 10 could have been a great film if it had developed the characters more, especially their complex relationships. As it is, it's mostly just lackluster. Don't get me wrong, I still love most the sequels but it's almost sad to think how much more they could be...
Bluth managed to struck the perfect tone for the universe. I kinda still think of it as a dying world with a few sanctuaries here and there to support life. Of course the sequels showed that things are getting better and I don't really know which vision is better. What the series really needs is a sense of threat in the Mysterious Beyond. I mean, in the Meadow of Jumping Waters the Gang just marches out of the Valley without good reason? When they know there are sharpteeth? Hmm... There was this fanfic Journey of a Forgotten Past that sums up what I mean with this comment.
Of course, the sharpteeth aren't the only thing that needs to be improved. The spiritual part of the first film is something I've been wanting to see in the sequels. LBT isn't really a supernatural series but a few nods to that direction could work. What I don't like to see is the Rainbowfaces returning. They worked well in one film but if they return and let everyone know that they're aliens...
